How to use the parallel port is not a Gambas problem, but a Linux kernel problem.

The kernel prevents any program from doing bad things like directly accessing hardware.
Instead, you must use devices, i.e. special files located in the /dev
directory, that allow you to access the hardware under kernel control.

You can access parallel port by using device files like /dev/lp0.../dev/lpN.
But if you need more controls, i.e. if you want to be able to write to x86
input/output ports, you use a device file named /dev/port.

2.2 An alternate method: /dev/port
Another way to access I/O ports is to open() /dev/port (a character device,
major number 1, minor 4) for reading and/or writing (the stdio f*() functions
have internal buffering, so avoid them). Then lseek() to the appropriate byte
in the file (file position 0 = port 0x00, file position 1 = port 0x01, and so
on), and read() or write() a byte or word from or to it.
Naturally, for this to work your program needs read/write access to /dev/port.
This method is probably slower than the normal method above, but does not
need compiler optimisation nor ioperm(). It doesn't need root access either,
if you give a non-root user or group access to /dev/port --- but this is a
very bad thing to do in terms of system security, since it is possible to
hurt the system, perhaps even gain root access, by using /dev/port to access
hard disks, network cards, etc. directly.
You cannot use select(2) or poll(2) to read /dev/port, because the hardware
does not have a facility for notifying the CPU when a value in an input port
changes.

So, as root, you can do what is explained above: opening /dev/port with
OPEN and read/write at a specific location.